When the first fully adult animal—Dolly, a sheep— was successfully cloned in 1996, it made headline news around the world. Since then, the practice of meddling in animal biology has speeded up exponentially. Humans have been tinkering with animals for centuries, of course— witness the incredible spectrum of dog breeds—but the new tools scientists have been adding to their toolboxes over the last two decades have taken that activity to a whole new level. Anthes not only reports on this phenomenon, she raises important questions about our responsibilities to animals, and about the impact of this experimentation on the living world.